The present invention relates broadly to electromagnetic interference (EMI) shielding enclosures, such as cases, housings, or parts thereof such as covers, for mobile, i.e., cellular telephone handsets and other electronic devices, and particularly to the manufacture of such enclosures as having a metallized shielding layer which is both conformal and corrosion resistant.
The operation of electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. As is detailed in U.S. Pat. Nos. 5,202,536; 5,142,101; 5,105,056; 5,028,739; 4,952,448; and 4,857,668, such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., between about 10 KHz and 10 GHz, and is termed xe2x80x9celectromagnetic interferencexe2x80x9d or xe2x80x9cEMIxe2x80x9d as being known to interfere with the operation of other proximate electronic devices.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other xe2x80x9ctargetxe2x80x9d devices from other source devices. Such shielding is provided as a barrier which is interposed between the source and the other devices, and typically is configured as an electrically conductive and grounded housing which encloses the device. The housing may be formed of a metal such as steel, aluminum, or magnesium, or alternatively, of a plastic or other polymeric material which is loaded with a metal or other electrically-conductive filler or which is provided with a metal or other conductive layer fastened, over-molded, spray painted, dip coated, clad, electrolessly or electrolytically plated, thermal or vacuum metallized, or otherwise generally applied or deposited across the interior surfaces of the housing. The conductive layer may be an electrically-conductive paint, a conductively-filled, molded elastomeric layer, a metal foil laminate, transfer, or liner, a metal plating, or a flame, arc, or other thermally-sprayed metal. A conductive gasket may be used to provide electrical continuity between the coating layers applied to the various mating housing parts. Such housings and methods are further described in commonly-assigned U.S. Pat. No. 5,566,055, in DE 19728839, U.S. Pat. Nos. 5,847,317; 5,811,050; 5,442,153; 5,180,639; 5,170,009; 5,150,282; 5,047,260; 4,714,623; and WO 00/29635; 99/43191; 99/40769; 98/54942; 98/47340; 97/26782, and in the following publications of the Chomerics Division of Parker Hannifin Corporation (Woburn, Mass.): xe2x80x9cCHO-SHIELD(copyright) Conductive Compounds;xe2x80x9d xe2x80x9cCHO-SHIELD(copyright) EMI Shielding Covers,xe2x80x9d Technical Bulletin 22, (1996); xe2x80x9cCHO-VER SHIELD(trademark) EMI Shielding Plastic Cover with Molded Conductive Elastomeric Gasket,xe2x80x9d (1999); xe2x80x9cCHO-SHIELD(copyright) 2052 Conductive Coating,xe2x80x9d Technical Bulletin 48, (2000); xe2x80x9cCHO-SHIELD(copyright) 2054 Conductive Coating,xe2x80x9d Preliminary Product Data Sheet, (2000); and xe2x80x9cCHO-SHIELD(copyright) 2056 High Performance Conductive Coating,xe2x80x9d Preliminary Product Data Sheet.
As to thermal spray processes, such processes have been used to apply metal and other coatings to a variety of substrates including metal, ceramic, and plastic. In general, by heating and accelerating particles of one or more materials to form a high-energy particle stream, thermal spraying provides a method by which materials supplied in wire or powder form may be rapidly deposited on a substrate. While a number of parameters dictate the composition and microstructure of the sprayed coating or article, the velocity and temperature of the particles as they impact the substrate often are the determining factors controlling the density and uniformity of the deposition.
One thermal spray process known as xe2x80x9cflame sprayingxe2x80x9d or xe2x80x9cmetallizingxe2x80x9d employs a combustion flame to spray metals and other materials in powder, wire, or rod form onto a metal, plastic, or ceramic substrate. A mixture of a fuel gas such as acetylene, kerosene, propylene, or hydrogen, and an oxygen-containing gas (oxy-fuel) are flowed through a nozzle and ignited within a combustion chamber or at the nozzle tip. The material to be sprayed, which typically is zinc, steel, bronze, molybdenum, aluminum, nickel, or aluminum, but which may be a ceramic, cermet, or a thermoplastic, is metered into the flame where it is heated, and is then atomized using compressed air or another gas to form a fine, molten spray which is propelled to the surface of the substrate. The droplets solidify upon contact with the substrate to form a coating. In a wire spray technique, the feedstock comprises a metal rod or wire which is passed axially or tangentially into the center of the flame front. In a powder spray variation, a metal powder is injected axially into the flame front by means of a carrier gas or by a gravity feed.
Conventional flame spraying is typically a low velocity thermal spray process in the subsonic range and usually produces coatings which have a high degree of porosity. More recently, and as is further described in WO 00/29635, the use of a high velocity oxygen flame (HVOF) has been proposed to improve coating density by accelerating the spray to Mach speeds.
Another thermal spray process known as xe2x80x9cplasma sprayingxe2x80x9d utilizes a high-velocity gas plasma to spray a powdered or other particulate material onto a substrate. To form the plasma, a gas such as argon, nitrogen, hydrogen, or helium, is flowed through the nozzle of a plasma spray gun having an anode and cathode. A potential difference is applied to develop an arc between the electrodes. Resistive heating by the arc causes the gas to ionized into a high-temperature, e.g., 10,000xc2x0 C. or higher, plasma stream. The powder or other particulate to be sprayed is entrained in the plasma and accelerated towards the substrate at a high velocity which may exceed Mach 1.
In another thermal spray technique known as xe2x80x9carc sprayxe2x80x9d which is further described in U.S. Pat. Nos. 3,546,415 and 4,668,852, two consumable solid or composite wires are employed as electrodes. An electric arc developed in an xe2x80x9carc zonexe2x80x9d between the tips of the wires causes the wires to heat and melt. As the wires melt, the arc is maintained between the tips by the continuous feed of the wires. The molten metal at the wire tips is atomized by a one or more streams of compressed air or another gas, and is accelerated by the gas to a substrate. The molten particles impacting the substrate rapidly solidify to form a coating. Alternatively, the arc spay technique may use non-consumable electrodes and instead, with metallized material being introduced into the arc zone as a powder. Conventional arc spray coatings are known to be generally dense and free of oxide.
Heretofore, a number of approaches have been tried to solve the problem of applying metallized coatings onto metal or plastic housings and other enclosures for electronic device. A preferred method would be economical and reliable, and further would produce EMI shielding coatings which are both corrosion resistant and conformal for covering ribs, walls, and other structures, irregularities, or discontinuities which may be molded or otherwise formed in the enclosure.
The present invention is directed to an electromagnetic interference (EMI) shielded enclosure, such as a case, housing, or a part thereof such as a cover, for mobile telephone handsets and other electronic devices. More particularly, the invention relates to an electrically-conductive, metallic or xe2x80x9cmetallizedxe2x80x9d coating layer for such enclosures which is conformally applied by means of a thermal spray process to an interior surface of a metal or plastic enclosure or enclosure part, such coating layer providing EMI shielding and also corrosion protection.
In an illustrative embodiment, the coating layer is formed by the electric arc spraying of tin, nickel, or an alloy thereof onto the surface of a enclosure part which may be diecast, stamped, machined or otherwise formed of a metal such as aluminum, steel. zinc, or magnesium, or which may be molded of a plastic material such as a polycarbonate (PC), acrylonitrile-butadiene-styrene (ABS), PC/ABS blend, polysulfone, acrylic, polyvinyl chloride (PVC), polyphenylene ether (PPE), polystyrene (PS), polyamide, nylon, polyolefin, or a copolymer or blend thereof. The coating layer so formed is self-adherent and conforms to ribs, wall, and other structures, irregularities, or discontinuities which may be formed enclosure part surface. An electrically-conductive gasket may be dispensed or molded onto the part as chemically-bonded or otherwise self-adhered to the coating layer. Alternatively, the gasket may be adhesively bonded onto the coating layer or mechanically fastened to the part over the coating layer.
Advantageously, the metallized coating layer may be formed by a electric arc thermal spray process which is well-characterized so as to be both economical and reliable. Moreover, by virtue of the use of tin, nickel, or an alloy thereof, the coating layer may afford the enclosure with increased corrosion resistance. The metallized coating layer also is compatible with convention EMI shielding gasket materials such as metal-filled urethanes, silicones, and other elastomeric resin which may be molded or dispensed on and self-adhered to the coating layer. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.